USB: OTG: no callback if mhl is not enabled

USB should not register the callback API if
MHL feature is not enabled from the boot param
options. This is consistent with the way the
regulators in USB driver are enabled for MHL.

Change-Id: I234b00e83e7eba068f7504c911991dda940dbf22
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>